The objective of Project 3 is to define novel molecular pathways that controi adipocyte differentiation and function. Elucidating signaling pathways that govern adipocyte differentiation and triglyceride storage in adipose tissue pathways is expected to uncover new opportunities for therapeutic intervention in human metabolic disease. During the current grant period, we discovered three previously uncharacterized proteins that control lipid metabolism in adipocytes (TLE3, PSPC1, and IDOL). Building on our work in the current grant period, we propose a series of molecular, cell biological, and mouse studies to investigate hypotheses regarding the roles of these novel regulatory factors in physiology and disease. Specific Aim 1 is to elucidate the mechanism of action of TLE3 in white versus brown adipose tissue phenotypic specification. We have discovered that mice overexpressing TLE3 in adipose tissue show profound alterations in adipocyte function, including a phenotypic shift from brown to white adipose tissue. Shifting white adipose tissue towards a more brown character is associated with improved metabolism and resistance to diabetes. A better understanding ofthe molecular pathways that control adipocyte phenotype would be highly significant and may have therapeutic implications. Specific Aim 2 is to determine the function and mechanism of action of the putative RNA-binding protein PSPC1 in adipocyte differentiation. PSPC1 was identified along with TLE3 in our initial high-throughput cDNA screen as a new promoter of adipocyte differentiation. Deciphering the mechanism of action of this novel factor is likely to uncover new modes of action for regulatory factors in the control of adipocyte gene expression. Specific Aim 3 is to define the role of the LXR-IDOL axis in triglyceride metabolism and lipid storage. Preliminary analysis of Idol knockout mice points to an unexpected function in the control of triglyceride metabolism and lipid storage. We hypothesize that LXR-dependent regulation of IDOL in adipose tissue is an important determinant of peripheral lipoprotein receptor activity and that IDOL provides a mechanism to link alterations in diet and sterol metabolism to lipid uptake and metabolism in these tissues. RELEVANCE (See instructions): Altered adipose tissue function or mass is associated with cardiovascular metabolic disease, underscoring the fundamental importance of adipocytes in metabolism. Elucidating signaling pathways that govern adipocyte differentiation and triglyceride storage in adipose tissue pathways is expected to uncover new opportunities for therapeutic intervention in human metabolic diseases (e.g., obesity, diabetes mellitus).